Hermetically sealed resistor



July 2 7, 1954 Filed Sept.- 16, 1952 K. BLACKBURN 2,685,016

HERMETICALLY SEALED RESISTOR 2 Sheets-Sheet 1 $5.514- 32E. E/B

oQi-AuR M 27, 1954 H. BLACKBURN 2,685,016

HERMETICALLY SEALED RESISTOR Filed Sept. 16, 1952 2 SheetsSheet 2OOOOOOO OOOOOOO OOOOOOO OOOOOOO OOOOOOO OOOOOOO OOOOOOO L1 E; I 2/INVENTOR.

l/sesaerz zncksueu otmm r am Patented July 27, 1954 HERMETICALLY SEALEDRESISTOR Herbert K. Blackburn, 1

The I-T-E Circuit Brea phia, Pa.

eadon, Pa., assignor to ker Company, Philadel- Application September 16,1952, Serial No. 309,860

7 Claims. 1

My invention relates to hermetically sealed resistors of the typewherein it is desirable to retain the inherent characteristics in itsoriginal construction.

In many electrical applications, it is essential that the resistivecomponent of the circuit maintain a constant resistive value, continuityand dielectric strength irrespective of temperature or atmosphericvariations.

Thus, it becomes necessary to provide a sealed coil to insure theexclusion of vapor, Water and other foreign matter to prevent the lossof these desirable characteristics and to prevent the absorption ofwater in the insulation and windings which would cause a reduction ofthe resistance of the coil.

It is well known in the art that failure of resistive components toretain proper continuity may be caused by either chemical action whenvapors and liquids are not excluded from the coil or by electricalaction when water containing ionized materials are absorbed.

Heretofore, the hermetically sealed resistors, designed to retain theabove mentioned characteristics, had several shortcomings anddisadvantages.

The prior art hermetically sealed resistors were constructed by windingthe resistive coil on a ceramic bobbin or coil form with the ends of thewire extending from respective ends of the coil form.

A circumferential edge at the outer ends of the coil form was then cutat a bias or chamfered to increase the adherence surface formetalization, as hereinafter noted.

After the bobbin was wound, a hollow cylindrical ceramic tube having adiameter slightly greater than the outside diameter of the barriers onthe coil form was positioned around the bobbin or the coil form.

The ends of the hollow cylindrical housing are metalized byelectro-chemical process to enable the sealing agent, such as leadsolder, to more readily adhere thereto.

After the hollow cylindrical housing is sealed over the coil form,terminal connectors are at-- tached to the respective ends of the coilform and the ends of the coil are attached thereto. Lead solder is thenpoured into the resulting cavity at each end to seal the hollowcylindrical housing, the terminal connectors at each end and the coilform. This construction thus results in a resistive component which issealed against adverse atmosphere conditions.

Many disadvantages were enconntered in this prior art hermeticallysealed resistor component. For example,

(1) The use of two diiferent materials, such as a ceramic and metalliclead solder which have different coefficiencies of thermal expansion,resulted in dangerous stres and strains of the unit when exposed totemperature extremes and thereby increased the possibility of fracturingthe seal;

(2) The ceramic coil form and housing have an inherent large thermocoefficiency of expansion which are undesirable characteristics for aresistive component of an electric circuit which is subjected to largetemperature changes;

(3) The use of an electrical conductive material such as lead solder asthe sealing agent could result in shorting in the windings of the coilin the event that molten solder was crossover flux and seep through;

(4) Since the ends of these prior art hermetically sealed resistorsconsisted primarily of solidified solder, it constitutes an electricalconducting surface. Hence, if the resistive component is mounted on edgeon a metallic base, the possibility of a short circuit is greatlyenhanced, Accordingly, it becomes necessary when mounting the resistoron end to provide additional insulation means;

(5) Due to the necessity of requiring metallic solder as a seal, theresulting resistive component is usually unduly heavy and bulky for usein electrical circuits having minimum space and weight requirements;

(6) The large number of separate and individual operations such aschamfering, grooving, metalizing, sealing, etc., are undesirable sincethey increase the cost of the component and result in excess handlingthereof;

(7) The use of a ceramic material, such as steatite which is frequentlyused, as a shell or housing for the sealed unit i highly undesirablesince ceramics of low porosity are undesirable from ceramics of highporosity except by means of complex and highly technical tests.

The hermetically sealed resistors of my invention overcomes all theaforementioned disadvantages and provides many additional advantageshereinafter noted.

My novel resistor is manufactured by placing the windings of theresistive coil on a resin coil form which is a nonhygroscopic compound.

The coil form is provided With a plurality of disc barriers andalternate sections of the coil are wound in opposite directions toreduce the in ductive eifect thereof.

The windings of the coil are positioned on a coil form in spacedrelationship to obtain a high ratio of air to wire to thereby obtain acushion effect to compensate for differences in coefficiency ofexpansion of the windings and the resin material of the coil form andseal.

After the windings of the coil are properly positioned on a coilwinding, the terminal connectors are attached to the respective endthereof. This subasseinbly, consisting of the winding form and itsassociated coil and terminal connectors, is then positioned in a moldand a liquid resin or sealing compound is molded there around. Eitherhot or cold pour or hot or cold cure liquid resin may be used.

The liquid material has the properties of adherence to the solidmaterial of the coil form, the ability to be converted to a solid at asafe temperature for the material of the coil form, high adherence tothe terminal connectors and a similar coefilciency of thermal expansionto the material of the coil form. Thus, the molding liquid may be thesame or different material to that used for the coil form.

By this method, I am able to obtain a hermetically sealed resistor whichretains the inherent characteristics of its original constructionirrespective of severe temperature and atmosphere changes andconditions.

Accordingly, the primary object of my invention is to provide ahermetically sealed resistor having a resinous seal which has the samethermal coefiicient of expansion as the winding form.

Another object of my invention is to provide a precision resistorrequiring no metaliz-ing such as silver, copper or tin for the sealing.

Still another object of my invention is to provide a hermetically sealedresistor in which the terminal connectors may be connected to the coilfrom before the coil is wound thereon to thereby greatly reduceunnecessary handling of the wound coil form.

A still further object of my invention is to provide a precisionresistive component in which the coil windings are positioned in uniquespaced relationship, resulting in a high ratio of air to wire to therebyprovide a cushioning effect to compensate for itself in the coe ciencyof thermal expansion of the coil and the resin seal.

Another object of my invention is to provide a hermetically sealedresistor which is light in weight and compact.

A still further object of my invention is to provide a resistorutilizing a minimum number of materials thereby susbtantially reducingundue stress and strains to eliminate the possibility of fracturing theseal.

Still another object of my invention is to provide a non-conductinghermetic seal for a resistive component.

Still another object of my invention is to pro vide a novel precisionresistor which is manufactured without a conducting seal so that it maybe mounted on its side or ends without the necessity of additionalinsulating material.

A still further object of my invention is to provide a low toleranceresistance in which the sealing material is a non-conductive material tothereby eliminate the possibility of shorting the windings of the coilas occurred in the prior art resistors which were sealed with aconductive material.

A still further object of my invention is to provide a hermeticallysealed resistor which is economical due to its simplicity of manufactureand the availability of the materials used.

A still further object of my invention is to provide a resistivecomponent completely surrounded by a non-porous material to insure theexclusion of vapor, water and other foreign materials.

Another object of my invention is to provide a hermetically sealedresistor which will retain the inherent characteristics of its initialconstruction.

Still another object of my invention is to provide a resistive componentwhich has constant resistance value, continuity and dielectric strengthirrespective of temperature and atmospheric changes.

Another object of my invention is to provide a non-hygroscopic coil formwhich prevents distorted windings, breaking of the hermetic seal andbreakdown of the dielectric.

Another object of my invention is to provide a precision resistor inwhich the coil form is made of the same non-hygroscopic compound as theouter seal.

A still further object of my invention is to provide a novel hermeticseal for a resistive component in which the compound is impervious toatmospheric conditions and fungus.

Still another object of my invention is to provide a resistor withrelatively low tolerance when subjected to the temperature cycling test(-65" C. to 95 C.) salt water immersion tests, humid cycle test andoverload test.

These and other objects of my invention will be apparent from thefollowing description when taken in connection with the drawings inwhich:

Figure 1a is a side view of the coil form used with my invention.

Figure 1b is an end view of Figure 1a taken along the line lblb.

Figure 2 is a side view of the terminal connector used with my novelresistor.

Figure 3a is a side view of the coil form shown in Figures 1a and lbwith the terminal connectors of Figure 2 attached to the respective endsthereof.

Figure 3b is an end view of Figure 3a taken along the line 3b-3b.

Figure 4 is a side view similar to Figure 3c showing the coil form withthe coil wound there- Figure 5 is a side view similar to Figure 4showing the wound coil form with an insulating tape wound on the outercircumference of the coil thereof, which may be used with my invention.

Figure 6a is a side view showing the assembly of a plurality of woundcoil units, as seen in Figures 4 or 5, assembled with stoppers andspacers in preparation for the molding operation.

Figure 6b is a side view of a spacer shown in Figure 6a.

Figure 6c is a side view of a spacer shown in Figure 6a.

Figure '7 is a perspective view showing the assembly of Figure 6amounted in a molding form.

Figure 8 is a cross-sectional view of the molding form shown in Figure'7.

Figure 9a is a cross-sectional view of the staggered spaced relationshipof the windings of my novel resistor.

Figure 9b is a cross-sectional view of the prior art method of parallelspaced relationship of the windings of a resistive coil.

Figure is a perspective view of my novel precision resistor.

Figure 11 is a cross-sectional view of the completed resistor of Figure10.

Figure 12 shows one method of mounting the completed resistor of Figure10 to a base.

Referring now to Figures 1a and 1b, the winding form H is made of asolid non-hygroscopic compound which may or may not be similar sealingcompound.

A winding form H is preferably made of a solid epoxy-type of heat-curingresin which may be sold commercially as a hysol. This type of compoundhas a high volume resistivity, high surfaced resistivity, low waterabsorption, high heat distortion and chemical inertness.

The winding form may be machined from a rod of solid resin material ormay be molded in the form shown in Figure 1.

The winding form i has a main shaft 20, with a hollow cylindricalportion 4!, and is provided with a plurality of discs or barriers l5, l6and ii which may or may not be integral therewith.

The barriers 5, l5 and H are placed on the winding form I! so thatportions of the electromagnetic field can be set up in opposition toreduce induction as will hereinafter be apparent.

The electrical barriers I5, 55 and H are provided with a radial notch Itto provide a path through which the wire of the coil may pass for thepurpose hereinafter described.

The shaft 29 of the winding form H is provided with a keyed flat section2| to receive a corresponding section 23 of the terminal connector 2|shown in Figure 2. The terminal connectors 2| are made of anon-corrosive tin-dipped material to be attached securely andpermanently to the winding form The keyed flat section 23 of theterminal connector 2! engages the keyed surfaces 4| of the winding formH, as best seen in Figure 3b.

The terminal connector 2| is also provided with a notch section 28 toreceive the ends of the coil winding. In a preferred method ofmanufacture and construction, the terminal connectors 2| are attached tothe winding form 5 prior to the time that the coil is wound thereon, asseen in Figures 3a and 3b.

The terminal connector 2| may be temporarily secured to the winding formII by an adhesive compound which may be the same material as used forthe winding form El. Wire i then wound on the shaft 26 between thebarriers i5 and It in a clockwise or right hand direction. The left endof the wire is passed through the notch is of the barrier I5 and securedto the terminal connector 23 at the notch 23.

The right hand end of the coil wire is then passed through the notch I8of the barrier it and wound on the shaft 29 between the barriers I6 andI! in an opposite direction such as counterclockwise or a left handwinding. The right end of the wire of the coil 3| is then passed throughthe notch 8 of the barrier I! and secured to the ri ht hand terminalconnector 2| at the notch 28.

It will be noted that a small quantity of solder may be used to securethe respective ends of the coil 3li-3-| to the right and left handterminal connectors 2|.

It will be noted that Figure 3a shows a preferred embodiment of myinvention. In the example shown, the terminals 2| are connected to theends of the winding form H and extend radially therefrom. However, itwill be apparent to those skilled in the art that the terminal con--nector may be connected to extend longitudinally or parallel to the axisof a winding form I I.

It will also be apparent to those skilled in the art that the terminalconnectors 2| may be secured to the winding form after the wire has beenwound into the coils 3|l3| on the winding form As best seen in thecross-sectional view of Figure 9a, the wire of the coils 303| are woundin staggered spaced relationship to achieve a high ratio of air to wire.

Figure 91) illustrates the conventional spaced parallel winding ofresistive coils to illustrate a low ratio of air to wire.

With the staggered spaced relationship of the windings, as illustratedin Figure 9a, a cushioning effect can be obtained to compensate for thedifference in coefliciency of thermal expansions of the coil 30-3l andthe resin of the winding form H and the seal hereinafter described.

It will be noted that after the wire has been wound in the winding formI I, illustrated in Figure 4, insulating tape 60 may be woundconcentrically on the outside circumferences of the coil 3D3|, as seenin Figure 5. The tape at is more flexible longitudinally than laterallyand i noncorrosive. Hence, the insulating tape 56, which may be anacetate film cloth, serves as additiona cushioning means to compensatefor the difference in coefiiciency of expansion of the two materials ofthe coil and seal.

The semi-assembled unit of Figure 4 may be molded with a sealingcomponent either individ ually or in groups with other similarsemi-assembled units. One preferred method of applying the hermetic sealto a plurality of semi-assembled units of Figure 4 is shown in Figure 6.Insulating conducting wires are inserted in the hollow portion 4| of thewinding form H. Disc shaped stoppers 42 are provided on each side of thesemi-assembly of Figure 4 with spacers 43 positioned adjacent thereto.

The stoppers 42 and spacers 43, as seen in Figures 60 and 6b,respectively, are circular in form and have a diameter equal to theinside diameter of the mold 5|. The stopper 42 is provided with a radialprotrusion 6! which extends through the opening of the mold 5i toprevent overflow of the sealing compound from one resistor to another.

The completed unit, as seen in Figure 6a, has a pancaked assembly inwhich two stoppers 42 and spacers 43 are sandwiched between adjacentwinding forms, as shown in Figure 4. The disc shaped stoppers 42 serveto prevent the flow of resin to adjoining winding forms, serve as a sidesupport for the mold to insure a flat, smooth surface for the resistorand also serve as a mold release.

The spacers 43 serve to provide a means of insuring that the stoppers 42are parted to the axis 4| of the resistor to be seal d, also reduce thepossibility of blow holes by providing a large volume and surface todissipate the extreme heats generated by the hermetic reels, servepositioners for the winding bobbin H to insure that they are spacedconcentric with the inside circumference of the mold form 5|.

The support wire 46 serves as a common mounting for the sandwichedassembly of the winding forms H and their associated stoppers 4i.spacers 43. The wire 40 may be a rigid member having a mold releasesubstance applied to the external surface thereof. However, it has beenfar more desirable to make the unit All of flexible wire with aninsulating material on the outside thereof so that it will not adhere tothe liquid resin.

It has been found that the insulated wire 50 provides the furtheradvantage of providing a cushioning means to prevent creepage of liquidresin into the center hole ii of the Winding form H.

.As best seen in Figure 7, the pancaked assembly of Figure 6a isinserted in the cylindrical mold form iii of Figure 8. In a preferredembodiment, the mold form I, having a diameter corresponding to thediameter of the spacers 33 and stoppers a2, is cylindrical in form andhas a longitudinal slot 55. However, it is apparent that either theinterior or exterior of the mold form 5% may assume any desirableconfiguration.

The mold form 5! is preferably mad of a material which will not adhereto the liquid resin when it hardens. However, an a ternate method ofconstruction may be used whereby the mold form 5! may be made of anymold material wherein the interior of same is coated with a mold releasesubstance.

After th pancake assembly of Figure 6a is inserted in the mold 51, asseen in Figure '7, the liquid resin or sealing material is pumped intothe mold through the opening 55 to fill the circumferential areasurrounding the winding form l i.

It will be noted that the sealing mate ial may be either a hot or coldpour or hot or cold cure resin. The resin which is used as a sealingmaterial for my novel resistor may be the same material such as hysol ora different material from that used for the winding form l but hav ingthe same coefiicient or" thermal expansion. However, it must have theproperties of (1) high adhesion to the solid material of winding form ii, (2) ability to be cured at a safe temperature for the other materialused in the resistor such as the insulation of wire 40, the mold of thewinding form H, the mold of the terminal connector 2i and the smallamount of solder used to attach the end of the coil 3il-3l to the terinal connectors 2 i, (3) the same thermal coefficient of expansion asthe material of the winding form.

After the sealing material has been allowed to cure or set, the pancakassembly may be removed from the mold 5| and in turn, the resistors itand their associated spacers and stoppers s3 s2 removed from the wireform 40.

Figure is a perspective View illustrating the novel resistor of myinvention. The cross-sectional View of Figure 10, as seen in Figure 11,illustrates the sealing compound surrounding. the coils 36-3! and itsadhesion the winding form H.

Thus, it may be seen that the end results in a precision resistor inwhich a winding ii and the hermetic seal form an integral unit in whichthe coils 8I are completely embedded therein.

As heretofore noted, winding form I i and seal are preferably made ofthe same material in order to obtain identical or substantially similarcoefficiency of thermal expansion of the two A solid epoxy-type of heatcuring resin having superior heat stability chemical resist ance andelectrical properties and sold as l-l'ysol 5500, which is hot or acidset may be used for the solid material H. The epoxy-type resins toproduct which I am referring are essentially reaction products ofepichlorohydrin and Bis-phenol A,

but any polyphenol or polyhalohydrin may be used. It has been found thatthis type of epoxy resin with the addition of 25% slate makes an idealwinding form. This material combination of seven parts resin and threeparts hardener is cured by heating at 100 C. for two hours, 200 C. forfour hours, with corresponding temperatures for intermediate timeintervals.

An addition of 25% slate increases the heat distortion by approximately20 and the above described material results in a winding form which hasthe properties of high volume resistivity, high su face resistivity, lowwater absorption, high heat distortion and chemical inertness.

The resin material used for the seal is a cold or amine set type ofepoxy known as Hysol 6020 with the addition of five percent slate. Thismaterial, a combination of four parts resin to one part hardener iscured for twenty four hours at 20 (3., two hours at 60 C., withcorresponding temperatures for intermediate time intervals.

This sealing material has high adherence to the winding form and to themetal terminals, ability to be converted into a solid at a safetemperature for the remaining parts of the resistor and hassubstantially the same coeflicient 1 thermal expansion as the windingform ll.

Since the sealing material readily adheres to the solid winding form H,after it is cured, it is difilcult or impossible to determine the lineof joining between the seal and the winding form.

Figure 12- illustrates one preferred method for mounting my novelhermetically sealed resistor. For example, the resistor F0 may bemounted on its end and attached to a base plate l2 by means of screw Hwhich is screwed into the hollow opening 5% of the winding bobbin ii.

Accordingly, I have provided a novel precision resistor in which thesealing mat ial readily adheres to the winding form on WhlCl'l a coilhas been wound in a staggered spaced relation.

Thus, my novel resistor is composed of a minimum number of materials andis provided with adequate compensation to insure against damage of theseal due to unequal coefliciency of thermal expansion.

1' have further provided a precision resistor which is relatively simplein design, economical to manufacture and maintains its accuracy intemperature cycling tests (65 C. to 0.), salt water test, humidity cycletest and overload test.

In the foregoing, I have described my invention only in connection withpreferred embodirents thereof. Many variations and modifications of theprinciples of my invention within the scope of the description hereinare obvious. Accordingly, I prefer to be bound not by the specificdisclosure herein but only by the appending claims.

I claim:

1. A hermetically precision sealed wire wound resistor comprising awinding form consisting of a solid epoxy-type heat-curing resin andtwentyfive per cent slate, having resistor wire wound thereon and anenclosure therefor comprising a cold-set type of epoxy resin having fiveper cent slate, said enclosure being integral with said winding form andcompletely sealing said winding form.

2. A precision wire-wound resistor comprising a winding form consistingof a solid epoxy-type heat-curing resin, said winding form having aresistor wire wound thereon and a resin of substantially the samecomposition integral with and completely enclosing and sealing saidwinding form.

3. A precision resistor comprising a winding form consisting of a solidepoxy-type heat-curing resin and having added thereto twenty-five percent slate and having a resistor winding thereon, and a thermoplasticresin having substantially identical thermal coefficient of expansion asthat of the winding form integral with and completely enclosing andsealing said winding form.

4. A precision resistor comprising a winding form composed of a solidepoxy-type heatcuring resin, a resistor winding thereon and an enclosurecomprising a non-hygroscopic resin integral with said winding form andcompletely enclosing and sealing said winding form, said housing havingsubstantially the same thermal coeflicient of expansion as said windingform.

5. A precision resistor comprising a winding form composed of a solidepoxy-type heat-curing resin, a resistor winding thereon, and anenclosure comprising a non-hygroscopic resin integral with said windingform and completely enclosing and sealing said winding form, saidenclosure having substantially the same thermal coefiicient of expansionas said winding form, the enclosure being integral with the winding formthroughout the winding forms area and spaced therefrom only by theinterposed resistor winding on said form.

6. The method of constructing a precision resistor which compriseswinding a resistor on a winding form, comprising a solid epoxy-typeheat-curing resin, inserting said winding form in a mold and admitting athermal setting resin having the same co-efficient of thermal expansionas said winding form into said mold and permitting the thermal settingresin to cure for a period of twenty-four hours to two hours at twentydegrees centigrade to sixty degrees centigrade With correspondingtemperatures at intermediate time intervals.

7. The method of making a precision resistor comprising winding aresistor wire on a form consisting of a solid epoxy-type heat-curingresin containing about twenty-five per cent slate, inserting saidwinding form in a mold and admitting thereto epon resin containing aboutfive per cent slate, and curing said resinous mixture between 20 andcentigrade for a period ranging between two and twenty-four hours.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,265,821 Siegel Dec. 9, 1941 2,286,161 Rights et al June 9,1942 2,460,795 Warrick Feb. 1, 1949 2,500,449 Bradley Mar. 14,19502,547,405 Mitchell et a1 Apr 3, 1951 2,558,798 Thom July 3, 19512,586,609 Burle Feb. 19, 1952 2,668,867 Exstein Feb. 9, 1954 FOREIGNPATENTS Number Country Date 796,138 France Mar. 30, 1936

1. A HERMETICALLY PRECISION SEALED WIRE WOUND RESISTOR COMPRISING AWINDING FORM CONSISTING OF A SOLID EPOXY-TYPE HEAT-CURING RESIN ANDTWENTY-FIVE PER CENT SLATE, HAVING RESISTOR WIRE WOUND THEREON AND ANENCLOSURE THEREFOR COMPRISING A COLD-SET TYPE OF EPOXY RESIN HAVING FIVEPER CENT SLATE, SAID ENCLOSURE BEING INTEGRAL WITH SAID WINDING FORM ANDCOMPLETELY SEALING SAID WINDING FORM.